Summary The development of novel positron emission tomography (PET) tracers tailored to detect tau in the brain has opened the opportunity of using them to improve diagnostic accuracy in Alzheimer's disease (AD) and related tauopathies. Emerging data from early studies -including our own- on legacy postmortem material with the most validated thus far, [F-18]-AV-1451 (T807, Flortaucipir), have shown that this ligand binds with strong affinity to paired helical filament (PHF)-tau aggregates in AD brains, and those that form as a function of age, closely matching the stereotypical spatiotemporal progression of neurofibrillary tangles (NFT). In agreement with these observations, patients clinically diagnosed with AD dementia and mild cognitive impairment exhibit significantly higher in vivo [F-18]-AV-1451 retention than cognitively normal in regions expected to harbor an elevated burden of tau lesions in AD. But our data also suggest that AV-1451 is not exempt of pitfalls. AV-1451 has relatively low affinity for tau aggregates in non-AD tauopathies, and exhibits strong off-target binding to neuromelanin- and melanin- containing cells, and some weaker binding to hemorrhages. There is a great need for further [F-18]-AV- 1451 neuroimaging-pathologic correlation studies conducted on material from individuals scanned while alive. Several second-generation tau tracers have more recently been reported. The two that have garnered most attention and are largely considered to have most promise are [F-18]-MK-6240 from Merck Translational Biomarkers, and [F-18]-PI-2620 from Piramal imaging and AC immune. To date, very limited information is available about their binding properties though. Yet, these ligands are quickly making their way into observational studies and clinical trials. Thus, a comprehensive postmortem validation is critical for determining their usefulness for antemortem diagnosis and staging of AD and other tauopathies, and to understand exactly what PET positivity means in terms of neuropathological substrate. This project has been designed to 1) validate MK-6240 and PI-2620 in human brain tissue representing a wide spectrum of neurodegenerative diseases, to best understand exactly what MK- 6240 and PI-2620 PET signal implies at a cellular and molecular level; specifically we will test the idea that these novel ligands selectively target NFT and abnormal neuritic accumulations of tau, and 2) to perform detailed neuroimaging-pathologic correlation analyses on postmortem material from individuals who underwent [F-18]-AV-1451, [F-18]-MK-6240 or [F-18]-PI-2620 scans while alive; these studies are critical to evaluate the potential utility of these tracers for the reliable in vivo quantification of tau pathology and disease progression tracking. This project will develop the data necessary to guide the correct interpretation of tau neuroimaging in clinical settings, and to define the potential utility of these PET tracers when testing therapies aimed at decreasing or halting the progression of tau aggregation.